A marketing vendor is using Google Tag Manager to deploy our Tealium code.

[joseph_brown]

What are the potential issues this could cause?

[simon_browning]

The potential that tags won't get loaded in time before the user moves off the page is extremely high.

[son_hoang]

There are a lot of factors, as Simon pointed out the chained file requests can slow down tag firing. Although this would not be as big an issue with a single-page website. If they're using GTM they may also not have as robust a data layer as you may need. The issues can be minimal or numerous depending on what tags you're trying to fire with what data.